The formation of native disulfide bonds is required for the stability and function of a large number of proteins. The lumen of the endoplasmic reticulum (ER) has long been recognized as the primary site for disulfide bond formation in newly synthesized proteins. The ER provides an oxidizing environment favorable for disulfide bond formation however, the mechanism used to maintain the redox potential of the ER is poorly understood. A step toward addressing this question was recently made with the identification in S. cerevisiae of a novel lumenal ER glycoprotein Ero1p (ER oxidoreductin-1) that appears to be essential in setting the ER oxidation state. A conditional mutation in ERO1 prevents disulfide bond formation in the ER and causes an increased sensitivity to exogenously added reducing agents. The effects of mutant Ero1p can be bypassed with the addition of oxidizing compounds to the growth medium. Biochemical studies suggest that Erolp is an oxidase for thioredoxin-like proteins in the ER. The goal of this study is to expand our knowledge of the properties of Ero1p by completing a detailed analysis of its structure and function. Molecular biological, biochemical, and genetic approaches will be used, focusing on three specific aims: (1) Characterization of the requirements of Ero1p for oxidation and disulfide bond transfer. (2) Identification of components in the Ero1p oxidation pathway. (3) Determination of the mechanism of ER membrane association and retention of Ero1p.